Recently, techniques have been developed for forming a polarization-reversed region in a ferroelectric single crystal having a desirable nonlinear optical constant and an electro-optic coefficient. Apparatuses for these techniques have also been developed, as have devices using such ferroelectric single crystals. For example, optical devices, such as optical frequency modulators and wavelength converters, that use the ferroelectric single crystals with a polarization-reversed region are intensely studied.
Among known techniques for forming a polarization-reversed layer on a ferroelectric single crystal are those using ion beams or electron beams (See, for example, Patent Documents 1 and 2).
FIG. 13 is a diagram showing how a polarization-reversed layer is produced by one such technique.
Step S1310: A ferroelectric substrate 1310 having nonlinear optical effect is grounded. The ferroelectric substrate may be a LiTaO3 substrate that is a Z-plate.
Step S1320: A beam of positive ion 1320, accelerated by a predetermined acceleration voltage, is focused on the side of the ferroelectric substrate 1310 that has been positively polarized as a result of spontaneous polarization of the ferroelectric substrate 1310. The ion beam may be a Si2+ ion beam.
Step S1330: The surface of the ferroelectric substrate 1310 is scanned by the focused ion beam 1320 in a periodic pattern 1330.
According to this technique, a periodically polarization-reversed layer can be formed by using a focused ion beam, and the need to form a mask is eliminated. As a result, the contamination with impurities that occurs during the masking can be prevented and the masking process itself can be omitted. Not only can the ion beam form the polarization-reversed layer to a considerable depth into the ferroelectric substrate, but it can also form the polarization-reversed layer in a periodic pattern with short periodicity. Such a polarization-reversed layer can be used to produce highly efficient wavelength converters.    Patent Document 1: JP-A-06-186604    Patent Document 2: JP-A-06-342111